1. Field of the Invention
The present invention relates to a safe photosensitive composition not containing any harmful compound and, more particularly, to a photosensitive compound suitably usable in the manufacture of, e.g., a display device or an image pickup device and a method of manufacturing a display device or an image pickup device using this photo-sensitive composition. The present invention also relates to a safe photosensitive composition suitably usable in the fabrication of various electronic parts or circuit boards requiring finer processing and a pattern formation method using this photosensitive composition.
2. Description of the Related Art
Conventionally, resists have been widely used in the formation of patterns in the fabrication of various electronic parts or circuit boards, including semiconductor integrated circuits such as LSIs, which require a number of micropatterning techniques, or in the manufacture of display devices such as CRTs and LCDs, or image pickup devices such as CCDs.
Representative examples of the resists used in the micro-patterning are resists containing acrylic resin, e.g., poly (methylmethacrylate) and poly(trifluoroethyl-xcex1-chloroacrylate), resists containing quinonediazide-novolak resin and currently beginning to be used in the mass production of LSIs, and chemical amplification type resists consisting of an alkali-soluble resin, a dissolution inhibitor, and an acid generator as disclosed in Jpn. Pat. Appln. KOKAI Publication No. 63-27829. It is unfortunate that any of these resists has a problem in safety since the resist is generally dissolved in an organic solvent to prepare a resist solution by which a coating film is formed. Therefore, a coater must be equipped with a local ventilating means.
On the other hand, a water-soluble resist which can be coated in the form of an aqueous solution and developed with water is also known. One example of this water-soluble resist is a photosensitive composition using ammon dichromate or a large quantity of a diazo compound, as a sensitizing agent, and also containing casein or polyvinyl alcohol as a water-soluble resin. Unfortunately, ammon dichromate is a special management substance having toxicity, and a large quantity of a diazo compound is also very harmful to a human body. Consequently, this photosensitive resin also has problems in safety and environmental pollution.
Of the resists enumerated above, those used as the resists in the manufacture of display devices or image pickup devices are a mixture of ammon dichromate or a bisazide compound and a water-soluble resist, and a resist containing photosensitive polyimide or a quinonediazide-novolak resin. These resists, therefore, have the same problems as the resists used in the micropatterning.
In addition to these resists, a so-called slurry photosensitive composition is known as a light-controlling resist playing a certain role in achieving the performance of a photo-related device, e.g., a fluoroscent resist of a CRT, or a light-shielding resist or a color filter resist of a display device or an image pickup device. This slurry photosensitive composition contains a powder of, e.g., a fluorescent substance, a dye, or a pigment and is thereby imparted the corresponding function of any of these materials. However, this powder as the component of the slurry photosensitive composition scatters or absorbs exposure light, resulting in a significantly decreased transparency of the resist. This consequently degrades the image formation capabilities such as resolution and sensitivity.
It is an object of the present invention to provide two types of photosensitive compositions. The first photosensitive composition is a safe slurry photosensitive composition superior in image formation capabilities such as resolution and sensitivity and containing no harmful compound. The second photosensitive composition is a safe water-soluble photosensitive composition capable of being dissolved in water without using any organic solvent while maintaining a sufficient sensitivity as a resist, and containing no harmful substance.
It is another object of the present invention to provide a method of manufacturing various types of devices, particularly a CRT and an image pickup device, in which the manufacturing process is simplified and therefore high-quality products can be manufactured with a high yield.
It is still another object of the present invention to provide a pattern formation method capable of safely performing the process without using any special equipment such as a ventilator.
The first photosensitive composition of the present invention is characterized by containing i) a compound which generates an acid when irradiated with light or ionizing radiation, ii) at least one type of a resin with acid-crosslinkability or acid-decomposability, and iii) a powder.
The first photosensitive composition is a safe slurry photosensitive composition not containing any harmful substance such as a large quantity of a bisazide compound or a harmful metal ion. In addition, the image formation capabilities such as the resolution and the sensitivity of the first photosensitive composition are so excellent that they cannot be expected to be achieved in any conventional slurry photosensitive composition inferior in the resist transparency or the sensitivity during exposure.
The second photosensitive composition of the present invention is characterized by containing i) a compound which generates an acid when irradiated with light or ionizing radiation, and ii) at least one type of an acid-crosslinkable resin having repeating units represented by Formulas (1) and (2) below: 
wherein R1 represents a monovalent organic group.
The second photosensitive composition can be dissolved in water without using any organic solvent, contains no or almost no harmful substance, and has a sufficient sensitivity as a negative type and chemical amplification type resist. Consequently, the amount of an additive such as an acid generator can be reduced. Also, the process can be done safely since no special equipment such as a ventilator is required in using this composition.
The device manufacturing method of the present invention is characterized by including the steps of i) forming a layer of the first photosensitive composition on a substrate, ii) exposing the layer in accordance with a predetermined pattern, and iii) heating the layer to form a fine pattern of the powder contained in the photosensitive composition.
This device manufacturing method can directly form a pattern having an optical function, unlike in conventional pattern formation methods. Consequently, the manufacturing process can be simplified. As a result, defective portions, such as defects of the pattern, produced during the manufacturing process can be reduced, and this makes it possible to manufacture high-quality products with a high yield.
The pattern formation method of the present invention is characterized by comprising the steps of i) coating the second photosensitive composition on a substrate, ii) irradiating light or ionizing radiation on the substrate coated with the photosensitive composition in accordance with a predetermined pattern, iii) performing chemical amplification by heating the irradiated substrate, and iv) developing the chemically amplified substrate with water.
In this method the pattern formation can be safely performed without requiring any special equipment such as a ventilator.
The first photosensitive composition of the present invention, i.e., a safe slurry photosensitive composition superior in image formation capabilities such as resolution and sensitivity and containing no harmful compound will be described below.
Examples of a compound which generates an acid upon irradiation with light, i.e., an acid generator, and is preferably water-soluble for use in the first photosensitive composition of the present invention are an onium salt, a quinonediazide compound (particularly orthoquinonediazides), and an organic halide.
Specific examples of the onium salt are diazonium salts, phosphonium salts, and sulfonium salts having fluoroborate anion, hexafluoroantimonate anion, hexafluoroarsenate anion, trifluoromethanesulfonate anion, paratoluene-sulfonate anion, and paranitrotoluenesulfonate anion, as counterions.
As the quinonediazide compound, salts of naphthoquinonediazidosulfonychloride and naphthoquinonediazidosulfonic acid can be suitably used.
The organic halide means a compound which forms hydrohalogenic acid. Examples are the compounds described in U.S. Pat. Nos. 3,515,552, 3,536,489, and 3,779,778 and West German Patent No. 2,243,621. More specifically, examples of the organic halide are the compounds described in U.S. Pat. No. 3,515,552, i.e., carbontetrabromide, tetra(bromomethyl)methane, tetrabromoethylene, 1,2,3,4-tetrabromobutane, trichloroethoxyethanol, p-iodophenol, p-bromophenol, p-iodobiphenyl, 2,6-dibromophenol, 1-bromo-2-naphthol, p-bromoaniline, hexachloro-p-xylene, trichloro-acetanilide, p-bromodimethylaniline, tetrachlorotetrahydronaphthalene, xcex1,xcex1xe2x80x2-dibromoxylene, xcex1,xcex1,xcex1xe2x80x2,xcex1xe2x80x2-tetrabromoxylene, hexabromoethane, 1-chloroanthraquinone, xcfx89,xcfx89,xcfx89-tribromoquinalidine, hexabromocyclohexane, 9-bromofluorene, bis(pentachloro)cyclopentadiphenyl, polyvinylidenechloride, and 2,4,6-trichlorophenoxyethylvinylether; and the compounds described in U.S. Pat. No. 3,779,778, i.e., hexabromoethane, xcex1,xcex1,xcex1-trichloroacetophenone, tribromotrichloroethane, and halomethyl-S-triazines. In particular, halomethyl-S-triazines such as 2,4-bis(trichloromethyl)-6-methyl-S-triazine and 2,4,6-tris(trichloromethyl)-S-triazine are favorable. A more favorable example of the organic halide is the compound disclosed in U.S. Pat. No. 3,987,037, which is substituted with vinylhalomethyl-S-triazine. This vinylhalomethyl-S-triazine compound is a photo-decomposable S-triazine having at least one trihalomethyl group and a group which conjugates with a triazine ring via at least one ethylenic unsaturated bond. The compound is represented by Formula (A) below: 
wherein Q represents bromine or chlorine. P represents xe2x80x94CQ3, xe2x80x94NH2, xe2x80x94NHR, xe2x80x94NR2, or xe2x80x94OR, R represents phenyl or a lower alkyl group having 6 or less carbon atoms, n represents 1, 2, or 3, W represents an aromatic ring, a heterocyclic ring, or a group represented by Formula (B) below: 
wherein Z represents oxygen or sulfur, and R1 represents a lower alkyl group or a phenyl group.
In Formula (A), the aromatic ring or the heterocyclic ring represented by W can be further substituted. Examples of the substituent are chlorine, bromine, phenyl, a lower alkyl group having 6 or less carbon atoms, nitro, phenoxy, alkoxy, acetoxy, acetyl, amino, and alkylamino.
Practical examples of the vinylhalomethyl-S-triazine compound represented by Formula (A) are the following compounds. 
Of the acid generators enumerated above, diazonium salts presented below are particularly preferable in that they dissolve well in water and have a high sensitivity to ultra-violet radiation. 
In the first photosensitive composition of the present invention, the mixing amount of the acid generator is not unconditionally defined because the amount depends upon the acid generation efficiency of the compound. However, the mixing amount is usually 0.01 to 20 parts by weight with respect to the total solid content, and preferably 0.2 to 10 parts by weight from the point of view of safety. If the mixing amount is smaller than 0.01 part by weight, it is difficult to obtain satisfactory effect of the mixing. If the mixing amount is larger than 20 parts by weight, the coating property, the safety, and the like factor may be degraded.
The first photosensitive composition of the present invention uses either a resin with acid-crosslinkability or a resin with acid-decomposability, when the acid-crosslinkable resin is used, the resultant photosensitive composition is a negative resist. When the acid-decomposable resin is used, a positive resist results.
Examples of the acid-crosslinkable resin are a homopolymer and a copolymer of vinyl alcohol; a copolymer of maleic anhydride; polyvinyl alcohol which is acetalized by, e.g., an aldehyde having a double bond; resins such as casein and cellulose having an OH group, a COOH group, or an acetal group with acid reactivity and a dehydration condensation property; a copolymer of polyamic acid, polyamino acid, or acrylic acid and a vinyl compound having a double bond on the side chain; a copolymer of vinyl alcohol and a vinyl compound having a double bond on the side chain; a melamine resin converted into methylol; an acrylic copolymer having a double bond on the side chain; an acrylic copolymer containing an epoxy group such as glycidyl-methyacrylate; a polymer having a double bond including a group such as allylether or ethylvinylether; and exposy resin; an epoxy resin which is modified to be alkali-soluble with acrylic acid or a carboxylic acid derivative; and a phenolic resin, such as a novolak resin, mixed with polyether, acetal, enolether, or acyliminocarbonate.
Of these resins, those having a carbonyl group or a phenolic hydroxyl group in the structure are more desirable since aqueous development is possible with these resins.
Also, of these resins an acetal resin having repeating units represented by Formulas (1) and (2) below is particularly preferably because of its high adhesion to a powder: 
wherein R1 represents a monovalent organic group.
In Formula (1), examples of the monovalent organic group represented by R1 are an alkyl group, an alkyl group containing a carboxyl group, an aromatic ring, and an aromatic ring having a substituent group.
The acetal resin containing repeating units represented by Formulas (1) and (2) can be obtained by, e.g., reacting polyvinyl alcohol with an aldehyde having a substituent represented by R1, in the presence of a catalyst such as an acid catalyst, thereby acetalizing the polyvinyl alcohol, i.e., converting the polyvinyl alcohol into the acetal. Examples of the aldehyde having a substituent represented by R1 are formaldehyde, acetaldehyde, propionaldehyde, butylaldehyde, isobutylaldehyde, valeraldehyde, isovaleraldehyde, hexanal heptamal, octanal, nonanal, decanal, paraformaldehyde, acrylaldehyde, methacrolein, crotonaldehyde, methacrylaldehyde, 2-methyl-2-butenal, propiolaldehyde, 2-butynal, chloroacetaldehyde, dichloroacetaldehyde, chloral, bromal, butylchloral, glycolaldehyde, lactoaldehyde, aldol, glyceraldehyde, glyoxylic acid, benzaldehyde, naphthoaldehyde, hydroxybenzaldehyde, and aldehyde crotonate.
In the acetal resin containing the repeating units represented by Formulas (1) and (2), the total amount of the repeating unit represented by Formula (1) is preferably 1 to 20 wt % of the entire resin. If the amount falls outside this range, the resultant photosensitive composition tends to degrade in, e.g., the sensitivity, the ratio of the residual film after exposure, and the solubility in water. The total amount of the repeating unit represented by Formula (2) is preferably 50 to 99 wt % of the entire resin. If the amount falls outside this range, the solubility of the resultant photosensitive composition upon water development decreases in some cases.
The molecular weight of the acetal resin containing the repeating units represented by Formulas (1) and (2) is preferably 1,000 to 300,000 as a weight-average molecular weight (Mw). If the molecular weight falls outside this range, the sensitivity or the solubility in an aqueous solvent may degrade.
Polyvinyl alcohol used as the material of this acetal resin is generally mass-produced by using vinyl acetate as a raw material. In the preparation of the acetal resin for use in the present invention, an unreacted portion of vinyl acetate can remain in polyvinyl alcohol. This unreacted portion of vinyl acetate, however, decreases the solubility of the resultant acetal resin in water. Therefore, the remaining amount of the unreacted portion is preferably 10 wt % or less of the entire resin.
These acid-crosslinkable resins can be used either singly or in the form of a mixture of two or more types of them. The resins can also be used in the form of a mixture with polyvinyl alcohol as a common water-soluble polymer, casein, or polysaccharides.
Examples of the acid-decomposable resin are a copolymer of tert-butylmethacrylate and methacrylic acid; a polyvinyl acetal resin with a high acetalization ratio; a resin crosslinked by vinyl ethanol; a resin having a tert-butyl group or a tert-butylester group, e.g., a tert-butoxycarbonylated hydroxystyrene or novalak resin, a tert-butylcarbonylmethylated hydroxystyrene or novalak resin, and a tert-butylated hydroxystyrene or novalak resin; and a trimethylsilylated phenolic resin. Of these resins, those having a carbonyl group or a phenolic hydroxy group in the structure are more desirable in that aqueous development is possible with these resins. These resins can be used singly or in the form of a mixture of two or more types of them. It is also possible to use the resins in the form of a mixture with resins of other sorts.
In the first photosensitive composition of the present invention, the content of these resin components is desirably 10 to 95 wt % of the total solid content in the composition. If the content is less than 10 wt %, the resolution or the sensitivity may degrade. If the content exceeds 95 wt %, the image formation capability may degrade.
As the powder contained in the first photosensitive composition of the present invention, a powder of a compound having an optical function such as a fluorescing property or a light shielding property or a powder of a compound having the function of an optical fiber can be suitably used as, e.g., a fluorescent substance, a dye, or a pigment. It is also possible to use a powder of a compound capable of improving the heat resistance or the strength as a structural material.
As the compound having a fluorescing property discussed above, either an inorganic or organic fluorescing compound can be used. As the inorganic fluorescing compound, it is possible to use salts normally used in fluorescent paints, fluorescent lamps, fluorescent substances for CRT, and pigments. Practical examples are inorganic acid salts such as MgWO4, CaWO4, (Ca,Zn)(PO4)2:Ti+, Ba2P2O7:Ti, BaSi2O5:Pb2+, Sr2P2O7:Sn2+, SrFB2O3.5: Eu2+, MgAl16O27: Eu2+, tungstate, and sulfate. Examples of the organic fluorescing compound are Acridine Orange, aminoacridine, quinacrine, an anilinonaphthalenesulfonic acid derivative, anthroyloxystearic acid, Auramine O, chlorotetracycline, cyanine dyes such as merocyanine and 1,1xe2x80x2-dihexyl-2,2xe2x80x2-oxacarbocyanine, dansychloride, derivatives such as dansylsulfoamide, dansylcholine, dansylgalacside, dansyltrizine, and dansylchloride, diphenylhexatriene, eosine, xcex5-adenosine, ethidiumbromide, fluorescein, formycine, 4-benzoylamido-4xe2x80x2-aminostilbene-2, 2xe2x80x2-sulfonic acid, xcex2-naphthyl triphosphoric acid, and oxonol dye, a parinaric acid derivative, perillene, N-phenylnaphthylamine, pyrene, Safranine O, fluorescamine, fluoresceinisocyanate, 7-chloronitrobenzo-2-oxa-1,3-dizaol, dansyladilizine, 5-(iodoacetamidoethyl) aminoaphthalene-1-sulfonic acid, 5-iodacetamidofluorescein, N-(1-anilinonaphthyl4)maleimide, N-(7-dimethyl-4-methylcoumanyl)maleimide, N-(3-pyrene)maleimide, eosine-5-iodoacetamide, fluoresceinmercuryacetate, 2-[4xe2x80x2-(2xe2x80x3-iodoacetamide)] aminonaphthalene-6-sulfonic acid, and a Rhodamine derivative.
One example of the powder of a compound having a light shielding property is a light-absorbing pigment. Practical examples are carbon pigments such as carbon black, carbon refined, and carbon nanotube; metal oxide pigments such as iron black, cobalt blue, zinc oxide, titanium oxide, and chromium oxide; sulfide pigments such as zinc sulfide; phthalocyanine pigments; sulfates, carbonates, silicates, and phosphates of metals; and an aluminum powder, a bronze powder, and a zinc powder. Examples of the organic pigment are nitroso pigments such as Naphthol Green B; nitro pigments; azo and azolake pigments such as Bordeaux 10B, Lake Red 4R, and chromophthal red; lake pigments such as Peacock Blue Lake, and Phodamine Lake; phthalocyanine pigments such as Phthalocyanine Blue; threne pigments such as Thioindigo red and Indanthrene Blue; quinacridone pigments; quinacridine pigments; and isoindolinone pigments. In addition to these pigments, the first photosensitive composition may also contain dyes in order to increase the absorbance.
Examples of the heat-resistant structural material are oxides and nitrides of aluminum and silicon, a filler, silicon carbide, and carbon fiber.
In the first photosensitive composition of the present invention, the content of these powder components is desirably 1 to 98 wt % of the total solid component in the composition. If the content is smaller than 1 wt %, satisfactory addition effect, such as optical effects, can not be obtained. On the other hand, a content larger than 98 wt % may degrade the resolution or the sensitivity.
It is also desirable that the average particle size of these powders be as small as possible. The average particle size is normally ⅓ or less of a pattern to be formed, and preferably 100 xcexcm or smaller.
In addition to the three components described above, the first photosensitive composition of the present invention may also contain a water-soluble, photo-crosslinking agent, such as bisazide, as the fourth component. Furthermore, in addition to these components the composition may contain an additive in an amount by which the effect of the present invention is not degraded. One example of the additive is a sensitizer of the acid generator, such as a squalium dye, which is sensitive to specific wavelength, particularly 340 nm or higher. It is also possible, where necessary, to add a dispersant of a powder, such as a nonionic or ionic surfactant or an electrification agent.
Generally, the first photosensitive composition of the present invention is dissolved in water or an organic solvent before being used. From the point of view of safety, the use of a water as a solvent is preferred. In this case it is possible to add an alcohol solvent, such as ethanol or propanol, to the extent to which the safety is not degraded, in order to prevent gelation of the composition. Additionally, if a coater capable of local ventilation is used, freer copolymer compositions are possible by the use of an organic solvent such as cyclohexanone.
As will be discussed later, the image formation capability of the first photosensitive composition of the present invention depends upon the chemical amplification mechanism. That is, an acid is generated by the acid generator upon exposure, and when heated this acid diffuses to function as a catalyst of a crosslinking reaction or a decomposition reaction. For this reason, in this composition even a slight amount of basic ion causes a decrease in the sensitivity. Therefore, a care must be taken so that no basic ion is mixed not only during the composition preparation process but also during the manufacturing process of each component to be contained in the composition.
The first photosensitive composition of the present invention can be suitably used in the manufacture of display devices, such as CRTs or liquid crystal devices, or image pickup devices. In particular, the composition largely contributes to the formation of patterns having optical functions of these devices.
A method of forming a fluorescent pattern using the first photosensitive composition of the present invention will be described below by taking a CRT as an example.
First, an aqueous solution of the first photosensitive composition of the present invention or a slurry solution prepared by adding a small amount of alcohol to this aqueous solution is coated on a CRT in which a light-shielding portion is already formed. The coated solution is blown with hot air at 50xc2x0 to 120xc2x0 C., or heated and dried on a heating means such as a hot plate, thereby forming a resist layer. Although the thickness of this resist layer depends upon the application, it usually ranges between 5 and 25 xcexcm in the case of a CRT. If the thickness falls outside this range, the fluorescence intensity or the resolution may decrease. Thereafter, the resist layer is exposed to exposure ray passed through a shadow mask. As exposure rays used in the exposure, ultraviolet radiation (wavelength=430 nm to 250 nm) generated by a high- or low-pressure mercury lamp is desirable since the light source is highly versatile and easy to use. The exposure amount is normally one millijoule to 1000 millijoules/unit cm2. After the exposure, post exposure baking is performed at 60xc2x0 to 150xc2x0C. A pattern is then formed by developing the resist layer with an appropriate developer such as water. If the solubility in the developer is low, alkali can be added to the developer. In the case of a CRT, the above pattern formation process is repeated three times for three fluorescent colors, R, G, and B. After all patterns are formed, aluminum is vapor-deposited on the rear surface to give the surface conductivity, and then tube sealing and removal of organic substances by heating are carried out. Thereafter, vacuum electrode sealing is performed to complete the CRT.
A method of forming a light-shielding pattern using the first photosensitive composition of the present invention will be described below by taking an LCD device as an example.
First, an aqueous solution of the first photosensitive composition of the present invention or a slurry solution prepared by adding a small amount of alcohol to this aqueous solution is coated on an LCD substrate on which a TFT element portion, such as an indium-tin-oxide (ITO) electrode or a data metal line, and a protective film are already formed. The coated solution is blown with hot air at 50xc2x0 to 120xc2x0 C. or heated and dried on a heating means such as a hot plate, thereby forming a resist layer. Although the thickness of this resist layer depends upon the application, it usually ranges between 0.5 and 1.5 xcexcm in the case of an LCD substrate. If the thickness falls outside this range, the light-shielding performance or the resolution may decrease. Thereafter, the resist layer is exposed to have a light-shielding pattern to be arranged between the elements through a mask. As exposure rays used in the exposure, ultraviolet radiation such as the g line or i line of a mercury lamp is commonly used. The exposure amount is normally several ten millijoules to several hundred millijoules/unit cm2. After the exposure, post exposure baking is performed at 60xc2x0 to 150xc2x0 C. A pattern is then formed by developing the resist layer with an appropriate developer such as water. If the solubility in the developer is low, an alkali can be added to the developer. After the pattern is formed as above, cell assembling and liquid crystal sealing is performed to complete the LCD device.
The first photosensitive composition of the present invention is a chemical amplification type resist. That is, and acid generated from the acid generator at the resist surface portion upon exposure is diffused vertically in the resist layer by heating. This acid acts as a catalyst for a crosslinking reaction or a decomposition reaction. If an acid-crosslinkable resin is contained in the composition, a negative pattern is formed. If an acid-decomposable resin is contained in the composition, a positive pattern is formed. As discussed earlier, this first photosensitive composition contains a powder. In conventional powder-containing photosensitive compositions, a portion which remains unirradiated with light exists on the back side of each powder particle, leading to deterioration in the sensitivity or the resolution. In the first photosensitive composition of the present invention, however, the acid catalyst penetrates by diffusion to the back side of each powder particle to which no light reaches. This makes it possible to efficiently bring about the crosslinking reaction or the decomposition reaction.
The second photosensitive composition of the present invention, i.e., a safe water-soluble photosensitive composition capable of being dissolved in water in the absence of an organic solvent while maintaining a sufficiently sensitivity as a resist, and not containing any harmful substance will be described in detail below.
As mentioned earlier, the second photosensitive composition of the present invention contains i) a compound which generates an acid when irradiated with light or ionizing radiation, and ii) at least one type of an acid-crosslinkable resin having repeating units represented by Formulas (1) and (2).
Of these components, the acid-crosslinkable resin having the repeating units represented by Formulas (1) and (2) is identical with the acetal resin having the repeating units represented by Formulas (1) and (2), which is discussed in detail in the explanation of the first photosensitive composition. Also, the repeating unit total amount, the mixing amount, and the molecular weight of the resin are the same as those explained for the photosensitive composition.
In the second photosensitive composition of the present invention, as the compound which generates an acid when irradiated with light or ionizing radiation, i.e., the acid generator, the compounds enumerated above as the acid generators usable in the first photosensitive composition can by directly used. In particular, diazonium salts represented by Formulas (3) to (5) below are preferable since each salt has a high solubility in water and a high sensitivity to ultraviolet radiation. 
The mixing amount and the like factor of the acid generator are the same as those of the acid generator used in the first photosensitive composition.
In addition to the above components, the second photosensitive composition of the present invention may contain an additive in an amount by which the effect of the present invention is not impaired. Examples of the additive are a water-soluble, acid-crosslinkable compound and a sensitizer of the acid generator, which is sensitive to a specific wavelength, particularly 340 nm or higher. More specifically, examples of the watersoluble, acid-crosslinkable compound are a melamine resin and a modified epoxy resin formed by conjugating a vinyl compound having carboxylic acid to an epoxy resin. A squalium dye is one example of the sensitizer of the acid generator.
When used as a resist, this second photosensitive composition functions as a negative resist in which a portion irradiated with light or ionizing radiation hardens. Although the mechanism of this reaction is uncertain, it is considered that the acid generated upon irradiation with light or ionizing radiation acts as a catalyst to improve efficiency of the reactions indicated by Reaction Formulas (I) and (II) below. 
The second photosensitive composition of the present invention is also a so-called chemical amplification type photosensitive composition containing a compound which generates an acid when irradiated with light or ionizing radiation. As discussed earlier, a photosensitive composition of this type makes use of the chemical amplification mechanism in which an acid is generated from the acid generator upon exposure with light or ionizing radiation and diffused by heating, and this acid acts as a catalyst to bring about a crosslinking reaction. As with the first photosenitive composition, therefore, the presence of a basic ion interferes with the crosslinking reaction, leading to a decrease in the sensitivity.
The water-soluble resin as the main component of the second photosensitive composition is an acetal-modified resin derivable from polyvinyl alcohol as described above. Generally, metal ions are mixed in polyvinyl alcohol during the manufacturing process. Consequently, a slight amount of metal ion is also contained in the acetal-modified resin as the product. A counter-ion of this metal ion deactivates an acid which plays an important role in the chemical amplification mechanism. This significantly decreases the sensitivity of the photosensitive composition which uses the chemical amplification mechanism. Therefore, any conventional chemical amplification type photosensitive composition primarily consisting of such a polyvinyl alcohol-based water-soluble polymer cannot be put into practical use, since it is impossible to fully utilize the sensitivity that the composition originally has.
The present inventors have found that the sensitivity of the chemical amplification type photosensitive composition primarily consisting of a polyvinyl alcohol-based water-soluble polymer is drastically raised by reducing the content of basic ions, particularly metal ions, and thereby the object of the present invention is achieved. That is a water-soluble photosensitive composition containing i) a water-soluble compound which generates an acid when irradiated with light or ionizing radiation and ii) a polyvinyl alcohol-based water-soluble polymer, in which the content of basic ions in the composition is 1,000 ppm or less with respect to the total solid component, is also included in the scope of the present invention. To distinguish from the second photosensitive composition, this photosensitive composition will sometimes be referred to as a third photosensitive composition hereinafter.
As the water-soluble compound which is contained in this third photosensitive composition and generates an acid when irradiated with light or ionizing radiation, the acid generator used in the first and second photosensitive compositions can be directly used. The polyvinyl alcohol-based water-soluble polymer includes polyvinyl alcohol and modified polyvinyl alcohol derived from polyvinyl alcohol.
If the quantity of basic ions contained in the photosensitive composition is 1,000 ppm or less, for example, as a quantity of metal ions, the composition can have a practical sensitivity. However, the quantity of basic ions is more preferably 500 ppm or less. Generally, the sensitivity of the photosensitive composition can be increased as the quantity of basic ions is decreased. Accordingly, it is possible to decrease the concentration of the acid generator to improve the safety.
In the third photosensitive composition, it is important that not only the acid generator contain less basic ion but the polyvinyl alcohol-based polymer contain less metal ion. As discussed above, polyvinyl alcohol as the material of the polyvinyl alcohol-based polymer contains metal ions mixed during the manufacturing process of the polyvinyl alcohol. Therefore, if this polyvinyl alcohol is used directly, it is impossible to obtain the third photosensitive composition of the present invention, so the content of metal ions must be reduced.
For example, the amount of metal ions contained in the polyvinyl alcohol-based polymer can be reduced by the use of a reagent containing as little metal ions as possible during the manufacturing process of polyvinyl alcohol as the material of the polyvinyl alcohol-based polymer, or by washing the polyvinyl alcohol or the polyvinyl alcohol-based polymer manufactured. More specifically, the metal ion content in the polyvinyl alcohol-based polymer can be reduced by, in manufacturing polyvinyl alcohol by hydrolyzing polyvinyl acetate, performing the hydrolysis by using a volatile acid and removing the acid by repeating decantation and condensation without neutralizing with an alkali, or by washing the polyvinyl alcohol or the polyvinyl alcohol-based polymer thus produced by using a solution such as oxalic acid.
The resin containing the repeating units represented by Formulas (1) and (2) is also a polyvinyl alcohol-based polymer. Therefore, by applying the constitution of this third photosensitive composition to the first and second photosensitive compositions of the present invention, i.e., by reducing the amount of basic ions contained in the first and second photosensitive compositions, the sensitivity can be further increased, and the content of the acid generator can be decreased. This makes it possible to obtain further safe water-soluble photosensitive compositions.
A method of forming a pattern using the second or third photosensitive composition will be described below.
First, a resist solution, i.e., an aqueous solution of the photosensitive composition or an aqueous solution added with a small amount of alcohol is prepared. The prepared resist solution is coated on a substrate, and the solvent of the resist is vaporized by heating to 50xc2x0 to 120xc2x0 C. by using a hot plate or the like. In this case any method commonly performed in this field of art can be used to coat the resist solution on the substrate. Generally, the thickness of the resist layer formed on the substrate after the solvent is vaporized preferably ranges from 0.05 to 25 xcexcm, although it changes in accordance with the application. If the thickness falls outside this range, the sensitivity may significantly decrease or the resolution may decrease.
Subsequently, exposure is performed by irradiating light or ionizing radiation on the resist layer formed on the substrate in accordance with a desired pattern. As the light or ionizing radiation used in the exposure, ultraviolet radiation, particularly ultraviolet radiation with a wave-length of 340 nm or more is preferable, since the light source is highly versatile and easy to handle.
After the exposure, post exposure baking is performed at a temperature of 60xc2x0 to 150xc2x0 C. Consequently, chemical amplification proceeds, and the resist is sensitized.
Lastly, the resist layer is developed with water to form a pattern. During this pattern formation, the solubility in water is in some cases slightly decreased depending on the type of polymer component used in the photosensitive composition, and this slows down the development. If this is the case, a slight amount of alkali can be added to water.
In the above method, a thin hard film also can be formed on the substrate by evenly exposing the entire surface of the resist layer, instead of doing the pattern exposure, and then performing post exposure baking. The film formed in this manner contracts when it is heated after the exposure, brining about a stretching effect. To obtain this effect, however, the post exposure baking must be done strongly. That is, it is preferable to perform, by using 5 parts by weight or more of the acid generator, exposure with an exposure amount large enough to cause the acid generator to completely react, and heating at a temperature of 140xc2x0 C. or higher after the exposure. By using the photosensitive composition of the present invention under these conditions, it is also possible to form an orientation film of a liquid crystal device, which can be oriented with light.
The pattern or the thin film formed by the above method by using any one of the first to third photosensitive compositions of the present invention is normally colorless and transparent. However, the pattern or the thin film can also be blackened by decreasing the transparency of the pattern or the film by performing post baking at a high temperature after the development. More specifically, it is only necessary to perform heating at about 140xc2x0 to 300xc2x0 C. for 1 to 60 minutes after the development. It is considered that this blackening is caused since an extreme dehydration reaction as indicated by Reaction Formula (III) below proceeds due to the heating at a high temperature and the action of an acid, and this carbonizes the film. 
This post baking is preferably performed in an anaerobic atmosphere or a nitrogen atmosphere. It is also possible to enhance the blackening reaction by increasing the concentration of the acid contained in the film by again irradiating the light or the ionizing radiation used in the exposure, or by supplying an acid into the film from a gas phase, a liquid phase, or some other polymeric phase, prior to performing the post baking.
The pattern or the thin film blackened in this way can be suitably used in a light-shielding portion of a display device or of a light receiving device. The resist layer formed by using any one of the photosensitive compositions of the present invention slightly exhibits conductivity when blackened. Therefore, the blackened pattern can also be used as a slightly conductive pattern.
Examples of the present invention will be described below. It is intended, however, that these examples are to merely explain the present invention, and the present invention is not limited to these examples.